Platinum L-zeolite catalysts for low-sulfur reforming were invented in the early 1980's. After about 10 years of intensive effort, and much research, low sulfur reforming was commercialized in the early 1990's. Progress toward commercialization required many discoveries. Two key discoveries were the criticality of ultra-low sulfur levels in the feed, and the impact of these ultra-low sulfur levels on reactor metallurgy, i.e., the discovery of the need to prevent coking, carburization and metal dusting. A preferred way to prevent coking, carburization and metal dusting utilizes a metal protective layer, especially one comprising tin.
While commercialization of ultra-low sulfur reforming was being pursued, a second generation of sulfur-sensitive platinum L-zeolite catalysts were being developed. These new catalysts are halided, for example, they are treated with freon or with ammonium halide salts. These catalysts allow operations at higher severity, tolerate a wide range of hydrocarbon feeds, have high activity and long life.
Our recent attempts to utilize this second generation of catalysts for ultra-low sulfur reforming resulted in an unexpected and undesired reduction in catalyst activity. After much research and experimentation, it was discovered that these halided catalysts had been partially poisoned by the metal of the metal protective layer, specifically by tin, which had been used to prevent carburization and metal dusting of the reactor system surfaces. Somehow, some of this tin had migrated and deposited on the catalyst. In contrast, when conventional platinum L-zeolite catalysts are used for ultra-low sulfur reforming in a tin-coated reactor system, neither tin migration nor catalyst deactivation due to tin migration are observed. The cause of these problems has now been traced to low levels of volatile hydrogen halides that, under certain conditions, evolve from the catalysts themselves. These hydrogen halides apparently interact with tin-coated surfaces and can deactivate the catalyst.
Therefore, one object of the present invention is to reduce catalyst deactivation by metals derived from a metal-coated reactor system. Another object of the invention is to reduce catalyst contamination from a freshly metal-coated reactor system which would otherwise result in catalyst deactivation. This new process will also improve the reproducibility of catalytic operations, especially platinum L-zeolite reforming operations, since catalyst activity and life can be better predicted.
The use of metal coatings and metal protective layers, especially tin protective layers, in hydrocarbon conversion processes is known. These layers provide improved resistance to coking, carburization and metal dusting, especially under ultra-low sulfur conditions. For example, Heyse et al., in WO 92/1856 coat steel reactor systems to be used for platinum L-zeolite reforming with metal coatings, including tin. See also U.S. Pat. Nos. 5,405,525 and 5,413,700 to Heyse et al. Metal-coated reactor systems are also known for preventing carburization, coking and metal dusting in dehydrogenation and hydrodealkylation processes conducted under low sulfur conditions; see Heyse et al., in U.S. Pat. No. 5,406,014 and WO 94/15896. In the '014 patent, Example 3 shows the interaction of a stannided coupon with hydrocarbons, methyl chloride and hydrogen at 1000 and 1200.degree. F. The coupon was stable to methyl chloride concentrations of 1000 ppm at 1000.degree. F., showing that the tin coating is stable to halogens at reforming temperatures.
The use of catalysts treated with halogen-containing compounds for catalytic reforming is also known. See, for example U.S. Pat. No. 5,091,351 to Murakawa et al. Murakawa prepares a Pt L-zeolite catalyst and then treats it with a halogen-containing compound. The resulting catalyst has a desirably long catalyst life and is useful for preparing aromatic hydrocarbons such as benzene, toluene and xylenes from C.sub.6 -C.sub.8 aliphatic hydrocarbons in high yield. Other patents that disclose halided L-zeolite catalysts include U.S. Pat. Nos. 4,681,865, 4,761,512 and 5,073,652 to Katsuno et al.; U.S. Pat. Nos. 5,196,631 and 5,260,238 to Murakawa et al.; and EP 498,182 (A).
None of these patents or patent applications disclose any problems associated with metal-coated reactor systems. Nor are they concerned with the problems associated with halided catalysts, especially platinum L-zeolite reforming catalysts interacting with metal coatings, such as tin coatings.
They neither teach the desirability nor the need for a catalyst pretreatment step to remove volatile halide acid(s), especially not prior to catalyst loading or prior to hydrocarbon processing. Indeed, the art teaches the advantages of combining one of the preferred coating metals--tin--with a reforming catalyst, specifically with a platinum L-zeolite catalyst. U.S. Pat. No. 5,279,998 to Mulaskey et al., teaches that activity and fouling rate improvements are associated with treating the exterior of the platinum L-zeolite catalyst with metallic tin particles having an average particle size of between 1 and 5 microns (tin dust). For example, Table I of the Mulaskey patent shows improved catalyst performance when metallic tin dust is combined with a platinum L-zeolite catalyst that has been treated with fluoride according to the process of U.S. Pat. No. 4,681,865.
In light of the above teachings, we were surprised to find a decrease in catalyst activity upon reforming in a freshly tin-coated reactor system using a halided platinum L-zeolite catalyst. (See Example below.)
We have now discovered that there are problems associated with using metal-coated reactor systems--especially freshly metal-coated systems--in the presence of halided catalysts, and we have discovered the cause of and solutions for these problems. Thus, one object of the present invention is to reduce catalyst contamination from a freshly metal-coated reactor system. Another object of the invention is to ensure that catalyst contamination is avoided, for example when replacing a conventional catalyst with a halided catalyst.